This invention relates to an improved device for generating acoustic shear waves in the earth, which is particularly adapted to seismic prospecting on land.
One known prior art device is adapted to generate acoustic shear waves which comprises essentially a target-member having a surface for coupling with the earth and a mass for striking the target-member in a direction substantially parallel to the earth surface.
The mass is connected to means a for applying thereto a velocity of predetermined amplitude thereto having a component parallel to the coupling surface and the direction of which can be oriented with respect to the target-member. The means for applying comprises for example, one arm supporting the mass at one of its ends and adapted to pivot at its opposite end with respect to the frame of a carrying vehicle, when actuated by driving means.
The target-member for example, is suspended from articulated arms on a carrying vehicle and is adapted to be vertically displaced from a position of transportation to a working position where it is in contact with the earth. In this working position, a portion of the weight of the vehicle bears on the target-member. For this purpose, the target-member includes tracks on which at least one axle of the vehicle with its wheels is caused to climb.
Moreover, there can be used a target-member having a coupling surface with the earth which is includes spikes or excrescenses which penetrate into the earth and contribute to the fastening of the device to the earth and to enhance the coupling and the power efficiency of the apparatus.
Such a device is described, for example, in U.S. Pat. Nos. 4,050,540 and 4,118,994.
The disadvantage of such coupling means is that it is not well adapted for use on very hard soils such as paths or roads where the excrescenses or protruding elements cannot penetrate without great difficulty. As a result of the impacts which are laterally applied thereto by the mass, the target-member undergoes translational motion which absorbs a part of the generated power. The result is a substantial decrease of the efficiency of the device.